Fix loading from Metro in Bridgeless mode

[PeteTheHeat]

Summary:
Problem Statement: A native module needs to call a function on ReactInstance (in this case loadScript). Typically, this is handled by the bridge.
Current Bridgeless Solution: Create a new protocol (in this case RCTJSScriptLoaderModule) which lets a block be passed in TM init to forward the method call to ReactInstance. This is the best thing I could think of right now.

Reviewed By: RSNara

Differential Revision: D22512748

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D22512748

[pull-bot]

	Messages
📖	📋 Missing Changelog - Can you add a Changelog? To do so, add a "## Changelog" section to your PR description. A changelog entry has the following format: [CATEGORY] [TYPE] - Message. CATEGORY may be: General iOS Android JavaScript Internal (for changes that do not need to be called out in the release notes) TYPE may be: Added, for new features. Changed, for changes in existing functionality. Deprecated, for soon-to-be removed features. Removed, for now removed features. Fixed, for any bug fixes. Security, in case of vulnerabilities. MESSAGE may answer "what and why" on a feature level. Use this to briefly tell React Native users about notable changes.
📖	📋 Missing Test Plan - Can you add a Test Plan? To do so, add a "## Test Plan" section to your PR description. A Test Plan lets us know how these changes were tested.
📖	📋 Missing Summary - Can you add a Summary? To do so, add a "## Summary" section to your PR description. This is a good place to explain the motivation for making this change.

Generated by 🚫 dangerJS against 03e1662

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	6,764,621	0
android	hermes	armeabi-v7a	6,432,282	0
android	hermes	x86	7,152,465	0
android	hermes	x86_64	7,042,373	0
android	jsc	arm64-v8a	8,935,969	0
android	jsc	armeabi-v7a	8,596,010	0
android	jsc	x86	8,766,922	0
android	jsc	x86_64	9,342,439	0

Base commit: da865b2

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	n/a	--

Base commit: da865b2

[react-native-bot]

This pull request was successfully merged by @PeteTheHeat in 448db78.

^{When will my fix make it into a release? | Upcoming Releases}